This invention relates to a method and apparatus for processing component signals to preserve high frequency intensity information.
FIG. 1 shows a conventional color video camera 2 and a conventional color cathode ray tube (CRT) display device 6. Camera 2 includes three linear sensors 4R, 4G and 4B, which generate respective color component signals R', G' and B'. The voltages of these three signals are proportional to the intensity of red, green and blue light respectively in the camera's focal plane. However, the intensity of light emitted by the screen of a conventional CRT is not linearly related to the voltage of the video signal that is applied to the electron gun of the CRT. In the case of a color CRT, the intensity of light emitted by the CRT is given by ##EQU1## where R, G and B are the driving voltages applied to the red, green and blue electron guns respectively, ** is the exponentiation operator and GAMMA is a constant (2.2 in the case of the NTSC system).
Because of this relationship between electron gun driving voltage and emitted light intensity, the video camera shown in FIG. 1 incorporates GAMMA correction circuits 5R, 5G and 5B, so that the red component signal R outputted by the camera is proportional to R'**(1/GAMMA), and similarly for G and B. The R, G and B color component signals provided by the camera may be used, with suitable amplification, to drive the CRT 7 directly, as shown in dashed lines, and the intensity of red, green and blue light emitted by the CRT would be proportional to R', G' and B' respectively. However, most color television standards, such as NTSC, PAL and RP125, encode visual information as luminance (Y), where EQU Y=0.299*R+0.587*G+0.114*B Eq. 2
and chrominance, or chroma (CR and CB). In the NTSC system, CR is equal to R-Y and CB is equal to B-Y. Therefore, video camera 2 includes a resistive encoding matrix 8 that converts the R, G and B component signals to luminance and chroma component signals and the display device 6 includes a decoding matrix 9 that receives the Y, CR and CB signals and reconstructs the R, G and B component signals therefrom.
The NTSC standard prescribes a bandwidth of 4.2 MHz for the Y component signal and a bandwidth of up to 1.2 MHz for the chroma component signals. This difference in bandwidth reflects the fact that the human eye is more sensitive to high frequency changes in intensity than to high frequency changes in brightness. In order to be able to generate a luminance component signal that fully utilizes the available bandwidth of the NTSC system, the primary color component signals must have a bandwidth of at least 4.2 MHz. Before combining the luminance and chrominance component signals to form a composite NTSC signal, the luminance component signal is limited in bandwidth to 4.2 MHz by a filter 10 and the chroma component signals are limited in bandwidth to 1.2 MHz by filters 11.
If R, G and B each range in value from 0 to 1, and R, G and B are each equal to 1, so that white light is emitted, Y is equal to 1 and the emitted light intensity E is equal to 1. However, because the emitted light intensity is a non-linear function of R, G and B, the luminance component, Y, is not sufficient to describe the intensity of the light emitted by the CRT. Thus, the intensity E is a function of Y, CR and CB and a given Y value will result in a higher intensity E when combined with large chroma values (large absolute values for CR and/or CB) than when combined with small chroma values. For example, a saturated full brightness red (R =1, G =0 and B =0) has a Y value of 0.299 and provides an intensity value E of 0.299, whereas a gray for which R=0.299, G=0.299 and B=0.299 also provides a Y value of 0.299 but results in an intensity E of 0.299**GAMMA, or 0.070 for GAMMA=2.2.
The non-linearity of the relationship between emitted intensity and R, G and B, and the consequence that E is a function of CR and CB as well as of Y, gives rise to a problem when the chroma component signals are filtered to a lower bandwidth than the luminance component signal, such as when preparing to combine the signals to produce an NTSC composite signal.
If color component signals R, G and B are used to drive a high resolution CRT display, and the value of R within a selected area of the field is 1 and elsewhere it is 0 and the values of G and B are 0 throughout the field, so that the CRT displays an area of saturated red against a black background, the peak value of Y is 0.299 and the peak value of E is 0.299. If these color component signals are converted to the NTSC standard, and the area of the field that is red is a vertical line that is at least as wide as allowed by the bandwidth of the chroma channels, the values of Y and E within the area of the red line are the same as in the case of the high resolution display. The peak values of R-Y and B-Y are 0.701 and -0.299 respectively. If, however, the line displayed on the high resolution CRT display was as narrow as allowed by the luminance channel bandwidth, the chroma filters employed in converting the color component signals to the NTSC standard would spread out the chroma signals by a factor of three or so, and reduce the peak values of the chroma signals by the same factor. Accordingly, while the peak value of Y is still 0.299, the peak values of R-Y and B-Y are 0.234 and -0.100 respectively, and the peak value of E is 0.095, or only about one-third of the intensity value for the wider line. This problem of reduced intensity is not limited to the case in which the signals are filtered in the horizontal direction, and arises also with vertical filtering, for example when component signals for driving a high resolution display, which may have more than a thousand lines per frame, are converted to a broadcast television standard having only about 500 or 600 lines per frame. In the case of the PAL system, in which GAMMA is equal to 2.8, the problem is even more severe.
As typified by FIG. 1, it is conventional to limit the bandwidth of the luminance and chroma component signals by low-pass filtering these signals. This necessarily implies that the low-pass filtering operation takes place after the GAMMA correction has been effected. With most natural images, where saturation and contrast are low, this provides a satisfactory display. However, in the case of an image that contains fine detail (high spatial frequencies) of saturated color against a dark background, such as can be generated by a computer graphics system, the low-pass filtering in the GAMMA-corrected domain causes a reduction in the brightness of the displayed image.
U.S. Pat. No. 4,999,702 issued Mar. 12, 1991 to the assignee of this application shows that high frequency intensity information in GAMMA-corrected color component signals may be preserved when encoding these signals into chroma and luminance component signals by generating a linear luminance component signal and GAMMA-corrected chroma component signals and generating a GAMMA-corrected luminance component signal that is a function of the linear luminance component signal and the GAMMA-corrected chroma component signals.
In U.S. Pat. No. 4,999,702, a look-up table and interpolator are used for generating the GAMMA-corrected luminance component signal. The look-up table must be recalculated for each different value of GAMMA, and this can be a time consuming process. Further, the method and apparatus disclosed in U.S. Pat. No. 4,999,702 result in a loss of saturation for colored details displayed against a dark background.